This invention relates to fuel cells and, in particular, to fuel cells in which reforming of hydrocarbon fuel supply is carried out internally of the cells.
There have been proposed a variety of fuel cell systems in which reforming of hydrocarbon fuel supply to produce hydrogen fuel process gas is carried out internally of the fuel cells of the system. The cells used in these systems are typically high temperature fuel cells such as, for example, molten carbonate fuel cells.
The internal reforming reaction is endothermic and helps to ofset heat generated in the fuel cell and, thus, to reduce the load on the fuel cell cooling system. Typically, the fuel cell is set for maximum utilization of process gas and realization of a desired operating temperature for the cell requires a cooling means usually in the form of a gas recirculation assembly. This assembly provides the needed cell cooling beyond that provided by the endothermic reforming reaction at the high process gas utilization.
The use of a cooling gas recirculation assembly, however, requires, a blower designed to accomodate the high temperatures of the fuel cell system. This, in turn, results in a high parasitic power requirement. Also, since the needed blowers are not conventional, they must be specially designed and manufactured at considerable expense. This increases significantly the cost of the overall fuel cell system, making it less attractive as an alternative to more conventional energy generation systems. Furthermore, use of process gas recirculation causes a higher pressure drop and also leads to undesired dilution of the reactants.
U.S. Pat. No. 4,187,795 discloses one fuel cell system in which the reforming reaction is used for achieving a desired operating temperature for the cell without requiring gas recirculation. In particular, in this system the reforming reaction is carried out in passages isolated from the process gas passages (i.e., isolated from the passages feeding the anode and cathode electrodes), and the flow of supply gas to these passages is controlled so that the cell operates at the desired temperature. The process gas flow in the process gas passages, in turn, is then controlled to obtain a desired electrical output. One drawback with the '795 patent system, however, is that to maintain desired fuel cell temperature, the system needs to reform a significantly larger quantity of fuel than can be utilized in the fuel cell.
While the '795 patent discloses that this excess fuel process gas can be used to feed the fuel process gas chamber of a further like internal reforming fuel cell placed in tandem with the first cell, this further cell also requires similar cooling as in the first cell. The further cell thus also needs to reform a significantly large quantity of fuel. As a result, the overall tandem arrangement likewise results in excess process gas and, in a practical system, would only utilize less than about 50% of the generated process gas.
It is, therefore, an object of the present invention to provide an internal reforming fuel cell system in which cooling is carried out via the internal reforming process, while the system is capable of utilizing at least 50%, but preferably, greater, amounts of generated fuel process gas.